5.0 ROADSIDE
BARRIERS

The
County may use guardrails to shield motorists from natural or man-made
obstacles located near the road. The primary purpose of the guardrail
is to prevent the vehicle, if it inadvertently leaves the road, to strike
a fixed object or travel a terrain feature that is considered more dangerous
than hitting the guardrail barrier. A guardrail is warranted if it reduces
the severity of the potential accident. In other words, if the consequences
of a vehicle striking a fixed object or running off the road are believed
to be more serious than hitting a traffic barrier, than a guardrail is
considered warranted. Guardrails are used typically to provide protection
from steep embankment heights, fixed objects, and pedestrians and bicyclists.
Table 8 outlines the guardrail warrants for non-traversable terrain
and roadside obstacles.

Table
8. Warrant Considerations for Guardrail Placement

Terrain
or Obstacle 1,2

Warrant

Bridge
Piers, Abutments, and Railing Ends

Shielding
generally required.

Boulders

A
judgment decision based on nature of fixed object and likelihood
of impact.

Culverts,
Pipes, Headwalls

A
judgment decision based on size, shape, and location of obstacle.

Cut
Slopes (smooth)

Shielding
not generally required.

Cut
Slopes (rough)

A
judgment decision based on likelihood of impact.

Ditches
(transverse)

Shielding
generally required if likelihood of head-on impact is high.

Embankments

A
judgment decision based on fill height and slope.

Retaining
Walls

A
judgment decision based on relative smoothness of wall and anticipated
maximum angle of impact.

Signs/Luminaire
Supports 3

Shielding
generally required for non-breakaway supports.

Trees

A
judgment decision based on site-specific circumstances.

Utility
Poles

Shielding
may be warranted on a case-by case basis.

Permanent
Water Bodies

A
judgment decision based on location and depth of water and likelihood
of encroachment.

Notes:


Shielding non-traversable
terrain or a roadside obstacle is usually warranted only when it is
within the clear zone and cannot practically or economically be removed,
relocated, or made breakaway, and it is determined that the barrier
provides a safety improvement over the unshielded condition.


Marginal situations, with
respect to placement or omission of a barrier, will usually be decided
by accident experience, with at the site or at a comparable site.


Where feasible, all sign
and luminaire supports should be a breakaway design regardless of their
distance from the road if there is reasonable likelihood of their being
hit by an errant motorist.

Source:
Roadside Design Guide, 1996 (p.5-5)

Embankment
height and side slope are basic factors considered in determining if a
guardrail is warranted. Table 9 lists the embankment height/side slope
situations that may warrant a guardrail. Encroachment occurring due
to the placement of the guardrail and the costs associated with the guardrail
will also be considered when using this table.

Table
9. Embankment/Side Slope Warrants for Guardrail

Side
Slope (H:V)

Embankment
Fill Height

1.5:1

3
ft or steeper

2:1

6
ft or steeper

2.5:1

9
ft or steeper

3:1

16
ft or steeper

Note:
1. Encroachment occurring due to the placement of the guardrail
and the costs associated with the guardrail will also be considered
when using this table.

Source:
Roadside Design Guide, 1996 (Figure 5.1, p. 5-3)

If
it is not immediately obvious whether the barrier or the unshielded condition
presents the greater risk, an engineering evaluation may be used. The
warrant will be established by using a benefit to cost analyses. The
evolution will take into account design speed, traffic volume, and costs
associated with the barrier or improvements. The evaluation will typically
look at three options.


Remove or reduce the area of
concern se that it no longer requires shielding,


Install an appropriate barrier,
and


Leave the area of concern unshielded.

The
use of guardrails to shield or protect drivers from roadside obstructions
is generally not cost-effective for roads with an average daily traffic
(ADT) less than 400 vehicles per day. A guardrail itself is a roadside
obstacle and a significant proportion of vehicles impacts with guardrails
produce injuries. The low frequency of collisions with guardrails and
the cost associated with maintaining guardrails makes it impractical for
using guardrails on low volume roads. Engineering judgment should be
used in determining locations where guardrail should be used due to the
potential of a severe accident due to departure from the road.